Say g'day on the sizzling sands of Australia and New Zealand

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You're in hot water — and it's okay — at Hot Water Beach, Coromandel Peninsula on New Zealand's North Island. The correct time to go is about two hours before or after low tide where you can soak in the naturally heated thermal mineral water. It's like spa-la-la on the white, sandy beach framed by lovely pohutukawa trees.

It's 85 million years ago, and the Earth's tectonic plates are slowly shifting, giving form to the islands and continents we recognize today. On the other side of the world, on the other side of the equator, New Zealand gradually separates from Australia, creating the Tasman Sea between the two landmasses.

Today, Australia is the world's largest island—and our only island-continent—and features 16,000 miles of coastline with nearly 11,000 beaches. That's the most in the world. New Zealand didn't do badly, either—the island-nation boasts more than 9,000 miles of coastal edging and more than 2,000 beaches.

But with great shorelines come great threats. Australia's waters are home to some of the world's most mystifying, and sometimes dangerous, creatures. "I don't encourage visitors to use unpatrolled beaches," says Dr. Andrew Short, a professor at the School of Geosciences, University of Sydney, where he directs the Coastal Studies Unit. He's also authored a seven-book series, "Beaches of the Australian Coast."

Compiling a list of Australia's sexiest beaches was a tall order. But it was a mission accomplished, thanks in large part to Dr. Short. He chooses several superbly sexy beaches, such as the Gold Coast's Rainbow Bay in Queensland and Bondi near Sydney in New South Wales. At Bondi, you can even swim in a seaside pool before dining at the iconic Icebergs. Cottesloe and Cable Beach get the sexy nod in Western Australia, while Mindil Beach in Northern Territory gets high marks for its Sunset Markets, which features 260 stalls. Other in-the-know Aussie sexy-beach-goers insisted that St. Kilda Beach in Melbourne, Victoria, be included.

The guys who write for Australia's Surfing Life magazine know a sexy beach when they see one. Editor Tim Fisher seconds Dr. Short's choices, and says Rainbow Bay is "right down the road from our office." There, famous Snapper Rocks lures sexy surfers with "the best wave in the world." Avid people-watchers are mesmerized by "chaos in the water, with Brazilians, champion surfers and learners all together. It can be ridiculously crowded and hilarious: eight on a wave."

Cottesloe, says Fisher, teems with "beautiful young people (BYPs) looking to meet other beautiful young people, plus you get those romantic ocean sunsets not seen on the East Coast." Mindil is just so NT, or Northern Territory, where sexy means tough, and Aussies who grow up in that region go for a swim and "just acknowledge sharks, jellies (jellyfish) and crocs as part of life on the beach: very Crocodile Dundee."

St. Kilda is the Melbourne equivalent of Bondi, but a bit "grubbier," explains Fisher, who says "sexy" has almost nothing to do with the beach itself, rather, that it's the closest beach from Australia's most cultured, cosmopolitan city. "Everyone goes, and it has backpackers, bars, clubs, and that fantastic, iconic pub, The Esplanade, a.k.a. 'The Espy.'" Like California's Venice Beach, St. Kilda has a promenade, good for skating and staring from behind your shades.

But Australia, clearly, is a big place. So we sought out more insider input for the second tier of beaches. They include Bells Beach and Ninety Mile Beach in Victoria; Byron Bay, Palm Beach, Manly Beach and Hyams Beach—with the world's whitest sand—in New South Wales; Four Mile Beach, Port Douglas and Whitehaven Beach, Whitsunday Island in Queensland and Margaret River in Western Australia.

Slideshow: Awesome Australia
It makes perfect sense that Australia is the home to both Rip Curl and Billabong, two brands that are synonymous with beach culture—nearly 80 percent of Aussies live within 31 miles of the shore. They flock like gulls to sun and surf and to read, play games, fish and fly kites, sip a latté at hip cafes or to just hang out. Sexy beach-bound Australians are likely making up for lost time. According to Surf Life Saving Australia, public "bathing" in Sydney was once restricted between the hours of 6 a.m. and 8 p.m. until some time between 1902 and 1905. That's when the city's seaside councils lifted their bans, clearing the way for sexy beaches to come into their own—even if it took a century.

The country's shores are lapped by the Pacific, Indian and Southern Oceans. There are no privately owned beaches here, but there are plenty of nude beaches, if going au natural is your thing.

New Zealand
In "100% Pure New Zealand," sexy beaches are not about babes in bikinis or guys in Speedos; it's more about spectacularly gorgeous and pristine landscape. For Kiwis, sex appeal is one of pure and basic unadulterated aesthetics, not of skimpy togs or a "froo-frooey" cocktail topped by a miniature paper umbrella. And besides, a locally made sauvignon blanc—or "savvy," in local dialect—is the typical tipple.

Onetangi Bay on Waiheke Island is a North Island must-do. Hop the 35-minute ferry from Auckland and head by car to a truly breathtaking spot that will leave the city buzz behind. You can see Great Barrier Island and Little Barrier from here — a memorable beach experience.

For much of the year, swimmers and surfers here tend to wear wetsuits, but that's a small sacrifice when the beach can be theirs—and theirs alone—for the entire day. Among the country's A-list beaches are the iron-ore-infused black sand at rugged Piha near Auckland and Hot Water Beach, where sand-soakers plop down together in a kind of thermally inspired "people soup." Nearby, New Chums Beach is another winner.

Slideshow: Explore New Zealand
As an American living in New Zealand, I advise everyone who visits Auckland to cross the Hauraki Gulf to Waiheke Island. There, Onetangi is picture-perfect, complete with traditional beach cottages, called baches (originally shortened from "bachelor pads"). March is a great time to go for the Beach Races, which feature horses, tractors and waiters stirring up the crystal sands. The South Island's Abel Tasman Park National Park offers numerous perfect crescent shapes of golden grains. See them first in a water taxi from Marahau, with Torrent Bay, Bark Bay, Tonga, Awaroa Bay, Totaranui and finally, Separation Point.

Because New Zealanders are known for their ruggedness, it may be impossible to simply sit on these sexy beaches. The surf, it seems, is always up. Greg Pope, an American surfer and 25-year New Zealand resident, rides his long board at the South Island's Dunedin, where he favors St. Clair Beach. But, he says, "I keep on finding more and more new beaches in New Zealand. There's always another one."